Method and apparatus for generating a phase dependent control signal

ABSTRACT

A phase detector generates a phase dependent control signal according to the phase relationship between a first and second clock signal. The phase detector includes first and second phase detector circuits receiving the first and second clock signals and generating select signals having duty cycles corresponding to the phase relationship between the clock edges of the first and second clock signals. The phase detector also includes a charge pump that receives select signals from the phase detector circuits and produces an increasing or decreasing control signal when the first and second clock signals do not have the predetermined phase relationship, and a non-varying control signal when the first and second clock signals do have the predetermined phase relationship. The control signal may be used to adjust the delay value of a voltage-controlled delay circuit in order to adjust the phase relationship between the first and second clock signals to have a predetermined phase relationship.

TECHNICAL FIELD

[0001] This invention relates to generating a control signal and, moreparticularly, to generating a control signal based on the phaserelationship between two input clock signals, and to memory devices andcomputer systems using such control signal generators.

BACKGROUND OF THE INVENTION

[0002] Conventional computer systems include a processor (not shown)coupled to a variety of memory devices, including read-only memories(“ROMs”) which traditionally store instructions for the processor, and asystem memory to which the processor may write data and from which theprocessor may read data. The processor may also communicate with anexternal cache memory, which is generally a static random access memory(“SRAM”). The processor also communicates with input devices, outputdevices, and data storage devices.

[0003] Processors generally operate at a relatively high speed.Processors such as the Pentium® and Pentium II® microprocessors arecurrently available that operate at clock speeds of at least 400 MHz.However, the remaining components of existing computer systems, with theexception of SRAM cache memory, are not capable of operating at thespeed of the processor. For this reason, the system memory devices, aswell as the input devices, output devices, and data storage devices, arenot coupled directly to the processor bus. Instead, the system memorydevices are generally coupled to the processor bus through a memorycontroller, bus bridge or similar device, and the input devices, outputdevices, and data storage devices are coupled to the processor busthrough a bus bridge. The memory controller allows the system memorydevices to operate at a clock frequency that is substantially lower thanthe clock frequency of the processor. Similarly, the bus bridge allowsthe input devices, output devices, and data storage devices to operateat a frequency that is substantially lower than the clock frequency ofthe processor. Currently, for example, a processor having a 300 MHzclock frequency may be mounted on a mother board having a 66 MHz clockfrequency for controlling the system memory devices and othercomponents.

[0004] Access to system memory is a frequent operation for theprocessor. The time required for the processor, operating, for example,at 300 MHz, to read data from or write data to a system memory deviceoperating at, for example, 66 MHz, greatly slows the rate at which theprocessor is able to accomplish its operations. Thus, much effort hasbeen devoted to increasing the operating speed of system memory devices.

[0005] System memory devices are generally dynamic random accessmemories (“DRAMs”). Initially, DRAMs were asynchronous and thus did notoperate at even the clock speed of the motherboard. In fact, access toasynchronous DRAMs often required that wait states be generated to haltthe processor until the DRAM had completed a memory transfer. However,the operating speed of asynchronous DRAMs was successfully increasedthrough such innovations as burst and page mode DRAMs, which did notrequire that an address be provided to the DRAM for each memory access.More recently, synchronous dynamic random access memories (“SDRAMs”)have been developed to allow the pipelined transfer of data at the clockspeed of the motherboard. However, even SDRAMs are typically incapableof operating at the clock speed of currently available processors. Thus,SDRAMs cannot be connected directly to the processor bus, but insteadmust interface with the processor bus through a memory controller, busbridge, or similar device. The disparity between the operating speed ofthe processor and the operating speed of SDRAMs continues to limit thespeed at which processors may complete operations requiring access tosystem memory.

[0006] A solution to this operating speed disparity has been proposed inthe form of a packetized memory device known as a SLDRAM memory device.In the SLDRAM architecture, the system memory may be coupled to theprocessor, either directly through the processor bus or through a memorycontroller. Rather than requiring that separate address and controlsignals be provided to the system memory, SLDRAM memory devices receivecommand packets that include both control and address information. TheSLDRAM memory device then outputs or receives data on a data bus thatmay be coupled directly to the data bus portion of the processor bus. Amaster clock signal transmitted to each memory device is used tosynchronize data transfer between the processor and memory device andalso serves as a basis from which to generate internal clock signalscoordinating internal memory operations.

[0007] One of the factors limiting the access speed of SLDRAM memorydevices is the speed at which the command buffer of each device canstore and process the command packets. The processing speed of thecommand buffer is dependent on the control of the relative timingbetween transmission of the command packets from the processor and aninternal clock signal ICLK of the memory device used to trigger a latchin the command buffer to capture the command signals. Both the commandsignals and the ICLK signal are delayed relative to receipt of thecommand packet on a command bus and a command clock signal CMDCLK.Furthermore, the amount of the delay is highly variable, and it isdifficult to control. If the delay of the internal clock signal ICLKcannot be precisely controlled, it may cause the latch in the commandbuffer to latch invalid command signals. Thus, the speed at whichcommand packets can be applied to the memory device is limited by thedelays in the memory device. Similar problems exist for other controlsignals in the memory device that control the operation of the memorydevice during each clock cycle, such as latching of data in the memorydevice and in a memory controller.

[0008] Consequently, the operation of a SLDRAM memory architecturenecessitates the generation of a sequence of clock signals havingpredetermined phases relative to a master clock signal. Phase-locked anddelay locked loops have been employed to ensure the precise phaserelationship between clock signals. In such a closed loop, there istypically a phase detector receiving two clock signals, and a voltagecontrolled delay circuit through which one clock signal passes. Thevoltage controlled delay circuit receives control signals from the phasedetector that are used adjust the variable delay value in order toestablish a predetermined phase relationship between the two clocksignals. For example, where the desired phase relationship between twoclock signals is zero degrees, the phase detector will detect any phasedifference between the two clock signals and generate a control signalthat is transmitted to the voltage controlled delay circuit. The delaycircuit will adjust the delay value according to the control signaluntil the clock signal passing through the voltage controlled delaycircuit is synchronized with the other clock signal. The clock controlcircuitry in an SLDRAM is described in greater detail in U.S. patentapplication Ser. Nos. 08/879,847, 08/890,055, 08/933,324, 08/994,461,09/146,716, and 09/150,079, which are incorporated herein by reference.

[0009] A single phase detector connected to a CMOS inverter has beenused as a means of providing a control signal to the above-describedvoltage controlled delay circuits. As shown in FIG. 1, clock signalsCLK1 and CLK2 are applied to two pulse generating circuits 11, 12, eachof which includes a NAND gate 16 receiving a respective clock signaldirectly and through three series connected inverters 18, 20, 22. Theoutput of each pulse generating circuit 11, 12 set and reset a flip-flop26 formed by cross-coupled NAND gates 28, 30. A single output of theflip-flop 26 is connected to the gates of an inverter 36 formed by aPMOS transistor 38 and an NMOS transistor 40. A current source 44supplies current to the source of the PMOS transistor 38, and a currentsink 46 draws current from the source of the NMOS transistor 40. Whenthe output from the flip-flop 26 is low, the PMOS transistor 38 isturned ON and the NMOS transistor 40 is turned OFF. In this condition, aconductive path is created for the current source 44 to couple currentto a capacitor 48. A control signal VOUT is generated by the capacitor48. When the current source 44 is applying current to the capacitor 48,the voltage of the control signal VOUT increases linearly. In thealternative case where the output from the flip-flop 26 is high, thePMOS transistor 38 is switched OFF and the NMOS transistor 40 isswitched ON. The current sink 46 is then coupled to the capacitor 48 todraw current from the capacitor 48. The voltage of the control signalVOUT then decreases linearly. As a result, the control signal VOUT has asawtooth waveform component.

[0010] The problem with using a single phase detector connected to aninverter 36, as shown in FIG. 1, is that even after the voltagecontrolled delay circuit has been adjusted so the clock signals have thepredetermined phase relationship, the circuit will nevertheless continueto generate a sawtooth ripple voltage at its output. The sawtoothwaveform component of the control signal VOUT is transmitted to thevoltage controlled delay circuit (not shown in FIG. 1), which is forcedto constantly adjust the delay value, and consequently, the phaserelationship between the two clock signals CLK1 and CLK2. The closedloop system will oscillate around a center-point and continue to “hunt”for the optimum control voltage value.

[0011] The result is a “phase jitter” imparted to clock signals used tolatch command and data signals. Although the phase jitter introduced bythe sawtooth ripple voltage may be acceptable in some applications, inhigh speed memory applications where the clock frequencies are high andthe need to control the phase relationship between clock signals iscritical, the clocks signals may fail to correctly latch command anddata signals.

[0012] To accommodate the problems associated with the sawtooth ripple,the memory system designer may relax the timing requirements of thememory system by slowing down the clock frequencies and reducing theoperating speed of the memory device. However, this approach defeats theprimary purpose of developing high speed memory systems. Therefore,there is a need for a phase detector that generates a control signalthat does not vary when the input clock signals have been adjusted to apredetermined phase relationship.

SUMMARY OF THE INVENTION

[0013] A phase detector used for generating a control signal based onthe phase relationship between two clock signals. The phase detectorincludes two phase detector circuits that each provide to a charge pumpor a phase dependent signal source select signals based on the phaserelationship of the clock signals. The charge pump receives the selectsignals and produces a current output signal according to combination ofthe select signals from the phase detector circuits. The current outputsignal may be converted into a control signal by connecting a capacitorto the output of the charge pump. Significantly, the phase detectorproduces a non-varying control signal when the two clock signals have apredetermined phase relationship. The use of two phase detectors and thecharge pump to generate control signals avoids the presence of asawtooth ripple voltage at the output of the charge pump.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014]FIG. 1 is a logic diagram of a conventional phase detector circuitand charge pump.

[0015]FIG. 2 is a block diagram illustrating a phase detector includingtwo phase detector circuits connected to a charge pump.

[0016]FIG. 3 is a logic diagram of the phase detector circuits of FIG. 2in accordance with an embodiment of the present invention.

[0017]FIG. 4, comprising FIGS. 4a-4 c, is a timing diagram showingseveral of the waveforms present in the phase detector circuits.

[0018]FIG. 5 is a logic diagram of the charge pump of FIG. 2 inaccordance with an embodiment of the present invention.

[0019]FIG. 6 is a block diagram of a clock generator circuit using anembodiment of the phase detector of FIG. 2.

[0020]FIG. 7 is a block diagram of a computer system using a pluralityof DRAMs, each of which includes the phase detector of FIG. 2.

DETAILED DESCRIPTION OF THE INVENTION

[0021] An embodiment of a phase detector 10 in accordance with thepresent invention is illustrated in FIG. 2. The phase detector includestwo single-to-dual signal converters 102, 104 that receive input clocksignals CLK1, CLK2, respectively, and produce complementary clocksignals CLK1*, CLK2* and non-complementary clock signals CLK1, CLK2based on the original input clock signals. The single-to-dual signalconverters may be implemented using a variety of designs known to oneskilled in the art. For example, an inverter and transfer gate havingthe same propagation delay connected in parallel will produce acomplementary and a non-complementary signal from an input clock signal.The CLK1, CLK1*, CLK2, CLK2* signals are transmitted to two phasedetector circuits 100, 101 that produce select signals OUT1, OUTI1*,OUT2, OUT2* and transmits them to a phase dependent signal source or acharge pump 200 via signal lines 106, 107, 108, 109, respectively. Thecharge pump 200 in turn generates an output current IOUT according tothe OUT1, OUT1* signals from the phase detector circuit 100, and theOUT2, OUT2* signals from the phase detector circuit 101. The IOUTcurrent may be converted into a control signal V₀ by capacitor 20connected to an output 280, and the control signal V₀ may be used toadjust the delay value of a voltage controlled delay circuit.

[0022]FIG. 3 illustrates an embodiment of the phase detector circuits100, 101 in greater detail. Each detector circuit 100, 101 consists oftwo signal transition detectors 110, 120 that generate a trigger pulseupon detecting a low-to-high transition of an input clock signal itreceives. Each detector circuit 100, 101 also includes a dual outputflip-flop 150 that is set and reset by the trigger pulses it receivesfrom the signal transitions detectors 110, 120. Each flip-flop 150 isformed by a pair of cross-coupled NAND gates 152, 154. The output fromthe phase detector circuits 100, 101 are applied to NAND gates 165, 166through inverters 161-164. Each NAND gate 165, 166 also receives arespective input signal from the flip-flop 150. The combination ofinverters 161-164 and NAND gates 165, 166 creates a buffer circuit thatwill cause the respective NAND gates 165, 166 to immediately switch uponreceiving a trigger pulse from the respective signal transitiondetector, as explained below.

[0023] Each phase detector circuit 100, 101 receives a pair ofactive-low control signals, SETA*, RSTA*, and SETB*, RSTB*,respectively. The SETA* and RSTA* signals are applied to the phasedetector circuit 100, and the SETB* and RSTB* signals are applied to thephase detector circuit 101. The control signals are generated by acontrol circuit (not shown), and are used to put each phase detectorcircuit 100, 101 into a predetermined state. Signal RSTA* is provideddirectly to the NAND gate 152 of the flip-flop 150 and the NAND gate 113of the signal transition detector 120, and signal SETA* is provideddirectly to the NAND gate 154 of the flip-flop 150 and the NAND gate 113of the signal transition detector 110. The RSTB*, SETB * signals aresimilarly provided to the respective NAND gates of the phase detectorcircuit 101.

[0024] To illustrate the operation of the control signals, consider theeffect the SETA*, RSTA* signals have on phase detector circuit 100.During normal operation of the phase detector circuit 100, the RSTA* andSETA* signals are both high. In this situation, the NAND gates 113 ofthe signal transition detectors 110, 120 behave as inverters, and theNAND gates 152, 154 behave as simple two input NAND gates. However, whenthe SETA* signal goes low, the output of the NAND gates 113 a and 154are forced high. Consequently, the NAND gate 118 a outputs a high to theNAND gate 152, and the NAND gate 118 b outputs a low that sets theflip-flop 150. The OUTI signal is then forced high and the OUT1* signalis forced low. In a similar manner, when the RSTA* signal goes low, theflip-flop 150 is reset so that the OUTI signal is forced low and theOUT1* signal is forced high. Control signals RSTB* and SETB* operate inthe same manner for the phase detector circuit 101 by forcing the OUT2,OUT2* signals to a predetermined state when active. To simplify theexplanation of the operation of the signal transition detectors 110,120, it will be assumed that the SETA*, RSTA*, SETB*, and RSTB* signalsare inactive (i.e., at a high signal level).

[0025] In operation, the CLK1, CLK2 signals are initially low therebyapplying a low signal directly to one input of the NAND gates 118 b, 118c and causing the inverters 116 a, 116 d to apply low signals to theother input of the NAND gates 118 a, 118 d, respectively. Thus, the NANDgates 118 a-d initially output a high signal. When the respective clocksignal goes high, e.g., the CLKI signal, the NAND gate 118 c outputs alow signal until the high signal has propagated through inverter 112 c,NAND gate 113 c, and through series inverters 114 c, 115 c, 116 c. Theinverter of 116 c then applies a low signal to the NAND gate 118 c,thereby causing the output of the NAND gate 118 c to again go high.Thus, the signal transition detector 110 outputs a low-going pulseresponsive to the CLK1 signal. The low-going pulse has a width equal tothe total propagation delay through inverter 112, NAND gate 113, andseries inverters 114, 115, 116. The signal transition detectors 110, 120in the phase detector circuit 100, and the signal transition detector120 in the phase detector circuit 101, each operate in the same mannerto output a low pulse responsive to the rising edge of the clock signalto which it is connected.

[0026] The low-going pulse from each of the signal transition detectors110, 120 sets or resets the flip-flops 150. More specifically, eachflip-flop 150 is set by each pulse from the respective signal transitiondetector 110, thereby causing the NAND gate 152 to output a high signaland the NAND gate 154 to output a low signal. Each flip-flop 150 isreset by each pulse from the respective signal transition detector 120,thereby causing the NAND gate 152 to output a low signal and NAND gate154 to output a high signal. The output of NAND gates 152, 154 are theninverted by NAND gates 165, 166, respectively, of the buffer circuit toprovide the OUT1, OUT1*, OUT2, OUT2* signals to the charge pump 200. Asa result, the OUTI signal is high during the period between the risingedge of the CLK2 signal and the falling edge of the CLK1 signal (i.e.,the rising edge of the CLK1* signal). In a similar manner, the OUT2signal generated by the detector circuit 101 is high during the periodbetween the falling edge of the CLK2 signal (i.e., the rising edge ofthe CLK2* signal) and the rising edge of the CLKI signal.

[0027] To illustrate the operation of the phase detector circuits 100,101, consider three situations: first, where CLK1 and CLK2 are in phase;second, where CLK1 is leading CLK2 by φ; and third, where CLK1 islagging CLK2 by φ.

[0028] The phase relationship when the CLK1 and CLK2 signals are inphase is illustrated in FIG. 4a. As explained above, the OUTI signalfrom the phase detector circuit 100 switches from low to high on therising edge of the CLK2 signal, and from high to low on the falling edgeof the CLK1 signal. Also, the OUT2 signal from the phase detectorcircuit 101 switches from low to high on the falling edge of the CLK2signal, and from high to low on the rising edge of the CLK1 signal.Since the CLK1 signal is shown in FIG. 4a as being in phase with theCLK2 signal, the duty cycles of the OUT1 and OUT2 signals are both 50percent, and the two signals will never be at the same logic levelsimultaneously.

[0029] Now consider the case where CLK1 is leading CLK2 by φ, as shownin FIG. 4b. When CLK1 is leading CLK2 by φ, the OUT1 signal from thephase detector circuit 100 and the OUT2 signal from phase the detectorcircuit 101 have duty cycles less than 50 percent, and may be at a lowlogic level simultaneously. Finally, consider the case where CLK1 islagging CLK2 by φ, as shown in FIG. 4c. When CLK1 is lagging CLK2 by φ,the resulting OUT1 and OUT2 signals from phase detector circuits 100 and101, respectively, have duty cycles greater than 50 percent. Thus, theOUT1 and OUT2 signals may be at a high logic level simultaneously.

[0030] The OUT1, OUT1*, OUT2, OUT2* signals are transmitted from thephase detector circuits 100, 101 on signal lines 106, 107, 108, 109,respectively, to the input of a charge pump, such as a charge pump 200,as shown in FIG. 5. The charge pump 200 includes a charging circuit 205,a current source 270, and a current sink 272. The function of thecharging circuit 205 is to direct the current of the current source 270and current sink 272 into, or out of the capacitor 20 (FIG. 2),respectively, depending upon the relative duty cycles of the OUT1 andOUT2 signals. Significantly, no current flows into or out of thecapacitor 20 when the CLK1 and CLK2 signals are in phase. Thus, theresulting control signal may have virtually no ripple when the phasedetector 10 (FIG. 2) is used in a voltage controlled delay circuit, asexplained above. A clock signal generated using the voltage controlleddelay circuit has significantly less phase jitter compared to a clocksignal generated by a voltage controlled delay circuit using the phasedetector of FIG. 1.

[0031] The charge pump 200 includes transistors 245-248 on the left legof the charging circuit 205 to form a compensation circuit 206 tocompensate for current and voltage changes in a current driving circuitformed by transistors 243, 244, 249, and 250 on the right leg of thecharging circuit 205. The compensation circuit 206 is provided so thatthe voltage across the charging circuit 205 is relatively constantduring operation, regardless of where the currents of the current source270 and the current sink 272 are being directed.

[0032] A voltage follower 260 is connected between the output 280 of thecharging circuit 205 and node 262 of the compensation circuit 206. Thevoltage follower 260 provides a current path from the current source 270to ground when current from the current sink 272 is being directed outof the capacitor 20. The voltage follower 260 also provides a currentpath from the current sink 272 to the positive supply when current fromthe current source 270 is directed to the output 280. As will beexplained in greater detail below, both of these situations occur wherethe CLK1 and CLK2 signals are not in phase. As a result, the currentthrough the charging circuit 205 is through two PMOS transistors and twoNMOS transistors when current is being directed into or out of thecapacitor 20, namely, the transistors 242, 244, 247, 251 or thetransistors 241, 245, 250, 252. Similarly, in the situations where nocurrent is being directed into or out of the capacitor 20, the currentthrough the charging circuit 205 is also through two PMOS transistorsand two NMOS transistors, namely, the transistors 242, 243, 248, 251 orthe transistors 241, 246, 249, 252. Consequently, the operating pointsof the active transistors will remain relatively constant, and anycapacitive charge pumping on the internal nodes of the charging circuit205 will be minimized. It will be appreciated by one ordinarily skilledin the art that the transistors of the charging circuit 205 must bescaled accordingly.

[0033] To illustrate the operation of the charge pump 200 in conjunctionwith the phase detector circuits 100, 101, consider again the threesituations that were described earlier: where CLK1 and CLK2 are inphase; where CLK1 is leading CLK2 by φ; and where CLK1 is lagging CLK2by φ.

[0034] As shown in FIG. 4a, when OUTI is low and OUT2 is high thecurrent provided by the current source 270 and sunk by the current sink272 bypasses the output 280 of the charging circuit 205 and simply flowsthrough the transistors 242, 243, 248, 251 (indicated in FIG. 4a as“0(A)”). Similarly, when OUT1 is high and OUT2 is low current flows fromthe current source 270 to the current sink 272 through the transistors241, 246, 249, 252 (indicated in FIG. 4a as “0(B)”). In either case, thecharging circuit 205 does not charge or discharge the capacitor 20 sothe voltage on the capacitor 20 remains constant.

[0035] As shown in FIG. 4b, when both the OUT1 and OUT2 signals are low,the charging circuit 205 directs the current provided by the currentsource 270 through the PMOS transistors 242, 244 to charge the capacitor20 (indicated in FIG. 4b as “I+”). A path for the current from thecurrent sink 272 is provided through the voltage follower 260 and theNMOS transistors 247, 251. During the time the OUT1 and OUT2 signals areat different logic levels, the charging circuit 205 does not charge ordischarge the capacitor 20 so the voltage on the capacitor 20 remainsconstant, as was previously explained (indicated in FIG. 4b as 0(A) or0(B)).

[0036] As shown in FIG. 4c, when both the OUT1 and OUT2 signals arehigh, the NMOS transistors 250, 252 provide a conductive path for thecurrent sink 272 to sink current from the capacitor 20 (indicated inFIG. 4c as “I−”). A current path for the current of the current source270 is provided through the voltage follower 260 and the PMOStransistors 241, 245. As mentioned before, whenever the OUT1 and OUT2signals are at different logic levels, the charging circuit 205 does notcharge or discharge the capacitor 20 so the voltage on the capacitor 20remains constant (indicated in FIG. 4c as 0(A) or 0(B)).

[0037] Any change in the control voltage V₀ depends upon whether currentis flowing into or out of the capacitor 20, as explained above. When theCLK1 and CLK2 signals are in phase, as illustrated in FIG. 4a, OUT1 andOUT2 never have the same logic level so no current flows either into orout of the capacitor 20. In contrast, when the CLK1 signal and the CLK2signal have different phases, OUT1 and OUT2 are both high or both lowfor a portion of each cycle. As illustrated in FIG. 4b, when the CLK1signal leads the CLK2 signal, OUT1 and OUT2 are low for more than 50percent of each cycle so that OUT1 and OUT2 are both low for a portionof each cycle. As a result, as explained above, current flows into thecapacitor 20, thereby increasing the control voltage V₀. Similarly, whenthe CLK1 signal lags the CLK2 signal as illustrated in FIG. 4c, OUT1 andOUT2 are high for more than 50 percent of each cycle so that OUT1 andOUT2 are both high for a portion of each cycle. As a result, currentflows out of the capacitor 20, thereby decreasing the control voltageV₀.

[0038] The discussion of the phase detector 10 has so far onlyconsidered the case where the CLK1 and CLK2 signals are adjusted so thatthey are approximately in phase. However, the phase detector 10 may bemodified to produce a control signal that adjust the CLK1 and CLK2signals to have a 180 degrees phase relationship. As shown in FIG. 3,the CLK1* and CLK2 signals are transmitted to nodes 95, 96 of the phasedetector circuit 100, while the CLK1 and CLK2* signals are transmittedto nodes 97, 98 of the phase detector circuit 101, resulting in a phasedetector that generates a non-varying control signal when the CLK1 andCLK2 signals are in phase. However, when the CLK1 and CLK1* signals arereconnected to the nodes 95 and 97, respectively, or the CLK2 and CLK2*signals are reconnected to the nodes 98 and 96, respectively, the phasedetector circuits 100, 101 transmit the OUT1, OUT1*, OUT2, OUT2* signalsto the charge pump 200 so that the phase detector 10 generates anon-varying control signal when the CLK1 and CLK2 signals have a 180degree phase relationship.

[0039] The current source 270 and the current sink 272 of the chargepump 200 may be of any current source circuit known in the art. In apreferred embodiment, a high-swing cascode current mirror, as describedin “CMOS Circuit Design, Layout, and Simulation,” published by IEEEPress, is used for both the current source 270 and the current sink 272.The use of this particular current source is meant for illustrativepurposes only, and is not intended to limit the scope of the presentinvention.

[0040] The charge pump 200 (FIG. 2) has been described with respect tothe embodiment illustrated in FIG. 5. However, an integrator circuitusing an operational amplifier may also be used for the charge pump 200.Such an integrator circuit is formed by coupling a capacitor across theoutput of the operational amplifier and the inverting input. The OUT1and OUT2 signals generated by the phase detector circuits 100 and 101are applied through two resistors of equal resistance to thenon-inverting input of the operational amplifier, and the OUT1* andOUT2* signals are applied through two resistors of equal resistance tothe inverting input. The resulting charge pump will generate increasingand decreasing control signals when the CLK1 and CLK2 signals are not inphase, and generate a control signal with virtually no ripple when theclock signals are in phase.

[0041] Shown in FIG. 6 is a block diagram of a clock generator circuitthat may be used in packetized DRAMs to provide a sequence of clocksignals that have predetermined phases relative to a master clocksignal. The clock generator circuit contains a first delay-locked loop301 and a second delay-locked loop 302, each having a phase detector 10of FIG. 2. A multiplexer 330 having a plurality of output lines coupledto respective clock drivers 314 a-n may be coupled to the firstdelay-lock loop 301 to couple one of the clock signals produced by themulti-tap voltage controlled delay circuit 310 to a clock outputterminal 316 for use, for example, with a latch 340 to latch commanddata CMD DATA in packetized DRAM. The multiplexer 330 couples the inputof each of the clock drivers 314 a-n to any one of the clock signalsproduced by the multi-tap voltage controlled delay circuit 310.

[0042] The first delay-locked loop includes a multi-tap voltagecontrolled delay circuit 310 and a first phase detector 10 a. Themulti-tap voltage controlled delay circuit 310 generates a sequence ofclock signals on output lines 312 a-312 n that are increasingly delayedfrom a first clock signal on line 312 a to a last clock signal on line312 n. Two of the clock signals, preferably the first and last clocksignals, are locked to each other using the delay-locked loop 301 sothat they have a predetermined phase with respect to each other. Forexample, the first clock signal on line 312 a and the last clock signalon line 312 n may be locked so that they are the inverse of each other,that is, the predetermined phase relationship is 180 degrees from eachother. Alternatively, the predetermined phase relationship could be 360degrees so the first and last clock signals are in phase. The firstphase detector 10 a compares the phase of the clock signals on lines 312a and 312 n and generates the first control signal as a function of thephase difference therebetween. The first control signal is provided tothe multi-tap voltage-controlled delay circuit 310 on line 311 to adjustthe relative delay between the clock signal on line 312 a and line 312n. The phase detector 10 a will continue to provide the first controlsignal until the first and last clock signals have obtained thepredetermined phase relationship.

[0043] Likewise, the second delay-locked loop 302 includes a secondvoltage controlled delay circuit 320 and a second phase detector 10 b. Asecond delay-locked loop locks a clock signal from the multi-tap voltagecontrolled circuit 310 to a master clock signal CMD CLK on line 305 sothat the increasingly delayed clock signals of the multi-tap voltagecontrolled delay circuit 310 have phase delays with respect to the CMDCLK signal. The clock signal from the multi-tap voltage controlledcircuit 310 is provided to an input of the second phase detector 10 bthrough a simulated multiplexer 317 and a clock driver 318. The relativephase delays of the simulated multiplexer 317 and the clock driver 318are nearly identical to that of the multiplexer 330 and the clockdrivers 314 a-n. Consequently, the phase detector 10 b will receive aclock signal having the same relative phase delay as a clock signaloutput by the clock drivers 314 a-n.

[0044] For example, the second delay-lock loop 302 may delay lock thefirst clock signal on line 312 a to the CMD CLK signal so that they havesubstantially the same phase, that is, the predetermined phaserelationship is zero degrees from each other. The voltage controlleddelay circuit 320 receives the CMD CLK signal and generates a referenceclock signal on line 322 having a delay relative to the CMD CLK signalthat is a function of a second control signal on line 321. The clocksignal on line 322 is provided to the multi-tap voltage controlledcircuit 310 and used to generate the sequence of increasingly delayedclock signals 312 a-312 n.

[0045] The second phase detector 10 b compares the phase of the CMD CLKsignal to the phase of the first clock signal on line 312 a andgenerates a second control signal as a function of the differencetherebetween. The clock signal provided to the phase detector 10 b isdelayed through the simulated multiplexer 317 and the clock driver 318approximately the same amount as the clock signals output by the clockdrivers 314 a-n. The second control signal is used to adjust the delayvalue of the voltage controlled delay circuit 320. The second controlsignal is provided by the second phase detector 10 b until the CMD CLKsignal and the first clock signal from the multi-tap voltage controlledcircuit 310 have obtained the predetermined phase relationship. Theclock generator circuit of FIG. 6 is described in greater detail in U.S.patent application Ser. No. 08/879,847, which, as mentioned above, hasbeen incorporated herein by reference.

[0046] A computer system using the phase detector 10 of FIG. 2 in eachof a plurality of packetized DRAMs 401 is shown in FIG. 7. Withreference to FIG. 7 the computer system 400 includes a processor 402having a processor bus 404 coupled to three packetized dynamic randomaccess memory or SLDRAMs 401 a-c. The computer system 400 also includesone or more input devices 410, such as a keypad or a mouse, coupled tothe processor 402 through a bus bridge 412 and an expansion bus 414,such as an industry standard architecture (“ISA”) bus or a Peripheralcomponent interconnect (“PCI”) bus. The input devices 410 allow anoperator or an electronic device to input data to the computer system400. One or more output devices 420 are coupled to the processor 402 todisplay or otherwise output data generated by the processor 402. Theoutput devices 420 are coupled to the processor 402 through theexpansion bus 414, bus bridge 412 and processor bus 404. Examples ofoutput devices 420 include printers and video display units. One or moredata storage devices 422 are coupled to the processor 402 through theprocessor bus 404, bus bridge 412, and expansion bus 414 to store datain or retrieve data from storage media (not shown). Examples of storagedevices 422 and storage media include fixed disk drives floppy diskdrives, tape cassettes and compact-disk read-only memory drives.

[0047] In operation, the processor 402 communicates with the memorydevices 401 a-c via the processor bus 404 by sending the memory devices401 a-c command packets that contain both control and addressinformation. Data is coupled between the processor 402 and the memorydevices 401 a-c, through a data bus portion of the processor bus 404.Although all the memory devices 401 a-c are coupled to the sameconductors of the processor bus 404, only one memory device 401 a-c at atime reads or writes data, thus avoiding bus contention on the processorbus 404. Bus contention is avoided by each of the memory devices 401 a-cand the bus bridge 412 having a unique identifier, and the commandpacket contains an identifying code that selects only one of thesecomponents.

[0048] The computer system 400 also includes a number of othercomponents and signal lines which have been omitted from FIG. 7 in theinterests of brevity. For example, as explained above, the memorydevices 401 a-c also receive a command or master clock signal to provideinternal timing signals, a data clock signal clocking data into and outof the memory device 401 a-c, and a FLAG signal signifying the start ofa command packet.

[0049] From the foregoing it will be appreciated that, although specificembodiments of the invention have been described herein for purposes ofillustration, various modifications may be made without deviating fromthe spirit and scope of the invention. For example, the signaltransition detector circuits 110, 120 have been described as generatinga negative trigger pulse upon detecting a transition of an input signal.However, using cross-coupled NOR gates instead of cross-coupled NANDgates for the flip-flop 150 allows the use of a signal transitiondetector that generates a positive trigger pulse. Also, the charge pump200 shown in FIG. 5 is described as generating a current output signalIOUT having a positive polarity when CLK1 is leading CLK2, and having anegative polarity when CLK1 is lagging CLK2. However, the connection ofthe OUT1, OUT1*, OUT2, OUT2* signals to the transistors 241-252 of thecharging circuit 205 may be modified so that the IOUT signal will have anegative polarity when CLK1 is leading CLK2, and have a positivepolarity when CLK1 is lagging CLK2. Accordingly, the invention is notlimited except as by the appended claims.

1. A phase detector for providing a control signal according to thephase relationship between a first and second clock signal, the phasedetector comprising: a first phase detector circuit having first andsecond input terminals coupled to receive the first and second clocksignals, respectively, and an output terminal, the first phase detectorcircuit producing a first select signal having a duty cycle according tothe phase relationship between a first clock edge of the respectiveclock signals; a second phase detector circuit having first and secondinput terminals coupled to receive the first and second clock signals,respectively, and an output terminal, the second phase detector circuitproducing a second select signal having a duty cycle according to thephase relationship between a second clock edge of the respective clocksignals; and a charge pump having first and second input terminalscoupled to the output terminals of the first and second phase detector,the charge pump producing a non-varying control signal responsive to afirst combination of logic levels of the first and second selectsignals, an increasing control signal responsive to a third combinationof logic levels of the first and second select signals, and a decreasingcontrol signal responsive to a third combination of logic levels of thefirst and second select signals.
 2. The phase detector of claim 1wherein the first combination of logic levels is the first and secondselect signals at different logic levels, the second combination oflogic levels is the first and second select signals at a high logiclevel, and the third combination of logic levels is the first and secondselect signals at a low logic level.
 3. The phase detector of claim 1,further comprising: a first logic circuit having an input coupled toreceive the first clock signal, and first and second output terminalscoupled to the first input terminal of the first and second phasedetector circuits, the first logic circuit providing non-complementaryand complementary clock signals responsive to the first clock signal toa respective output terminal; and a second logic circuit having an inputcoupled to receive the second clock signal, and first and second outputterminals coupled to the second input terminal of the first and secondphase detector circuits, the second logic circuit providingnon-complementary and complementary clock signals responsive to thesecond clock signal to a respective output terminal.
 4. The phasedetector according to claim 1 wherein the charge pump comprises anintegrator circuit.
 5. A phase detector for providing a control signalaccording to the phase relationship between a first and second clocksignal, the phase detector comprising: a first phase detector circuithaving first and second input terminals coupled to receive a respectiveclock signal and an output terminal, the first phase detector generatinga first select signal having a ratio of time at a first logic level andat a second logic level based on the phase relationship between a firstedge of the first and second clock signals; a second phase detectorcircuit having first and second input terminals coupled to receive arespective clock signal and an output terminal, the second phasedetector generating a second select signal having a ratio of time at afirst logic level and at a second logic level based on the phaserelationship between a second edge of the first and second clocksignals; and a charge pump having a first and second input terminalcoupled to the output terminals of the first and second phase detectorand an output terminal, the charge pump producing a changing controlsignal when the logic levels of the first and second select signalshaving a first predetermined relationship and a non-varying controlsignal when the logic levels of the first and second select signals havea second predetermined relationship.
 6. The phase detector of claim 5wherein the first predetermined relationship is when the logic levels ofthe first and second select signals have the same logic level and thesecond predetermined relationship is when the logic levels of the firstand second select signals have different logic levels.
 7. The phasedetector according to claim 5, further including a capacitor coupled tothe output terminal of the charge pump.
 8. The phase detector accordingto claim 7 wherein the first and second phase detector circuitscomprise: a first signal transition detector having an input terminalcoupled to receive a respective input clock signal and an outputterminal, the first signal transition detector producing a first triggerpulse signal in response to the respective input clock signal changinglogic states; a second signal transition detector having an inputterminal coupled to receive a respective input clock signal and anoutput terminal, the second signal transition detector producing asecond trigger pulse signal in response to the respective input clocksignal changing logic states; and a flip-flop having first and secondinput terminals coupled to the output terminal of the first transitiondetector and the output terminal of the second transition detector,respectively, and first and second output terminals coupled to arespective input terminal of the charge pump, the flip-flop setresponsive to the first trigger pulse signal and reset responsive to thesecond trigger pulse signal.
 9. The phase detector according to claim 8wherein the first and second signal transition detectors comprise: aNAND gate; and at least one inverter connected in a delay chain coupledbetween inputs of the NAND gate wherein the output of the delay chain isa complement of the input clock signal.
 10. The phase detector accordingto claim 8 wherein the first and second flip-flops comprise first andsecond NAND gates cross-coupled connected.
 11. The phase detectoraccording to claim 5 wherein the charge pump comprises: a first currentgenerator circuit coupled to a first reference voltage; a second currentgenerator circuit coupled to a second reference voltage; and a chargingcircuit coupled between the first and second current generator circuitsand having first and second input terminals coupled to a respectiveoutput terminal of the first and second phase detector circuits, and anoutput terminal coupled to transmit the control signal to the outputterminal of the charge pump, the charging circuit generating the controlsignal responsive to the first and second select signals from the firstand second phase detector circuits.
 12. The phase detector according toclaim 11 wherein the charging circuit comprises: first and secondswitches having a control gate and being coupled to the first currentgenerator circuit; a third switch having a control gate coupled to thecontrol gate of the first switch, and being coupled to the secondcurrent generator circuit; a fourth switch having a control gate coupledto the control gate of the second switch, and being coupled to thesecond current generator circuit; a first complementary pair of switcheshaving a node between the pair of switches coupled to the outputterminal of the charge pump, and a first and a second control gatecoupled together, the first complementary pair being coupled between thefirst and third switches; a second complementary pair of switches havinga node between the pair of switches, and a first and a second controlgate coupled together, the second complementary pair being coupledbetween the first and third switches and in parallel to the firstcomplementary pair of switches; a third complementary pair of switcheshaving a node between the pair of switches, and a first and a secondcontrol gate coupled together and to the first and second control gatesof the first complementary pair, the third complementary pair beingcoupled between the second and fourth switches; and a fourthcomplementary pair of switches having a node between the pair ofswitches coupled to the node of the second and third complementarypairs, and a first and a second control gate coupled together and to thefirst and second control gates of the second complementary pair, thefourth complementary pair being coupled between the second and fourthswitches and in parallel to the third complementary pair of switches,wherein each of the coupled control gates are coupled to a respectiveoutput terminal of the first and second phase detector circuits.
 13. Thephase detector according to claim 12 wherein the first and secondswitches are PMOS transistors, the third and fourth switches are NMOStransistors, and the first, second, third, and fourth complementarypairs of switches are CMOS transistors.
 14. The phase detector accordingto claim 11 wherein the first and second current generator circuits arecurrent mirror circuits.
 15. The phase detector according to claim 5wherein the charge pump comprises: a first current source coupled to afirst reference voltage to produce a first reference current; a secondcurrent source coupled to a second reference voltage to produce a secondreference current equal to the value of the first reference current; acurrent driving circuit coupled between the first current source and thesecond current source, and having an output terminal coupled to transmitthe control signal to the output of the charge pump, the current drivingcircuit receiving the first and second reference currents andtransmitting the control signal responsive to the first and secondselect signals from the first and second phase detector circuits; and acurrent compensation circuit coupled between the first current sourceand the second current source in parallel to the current drivingcircuit, the current compensation circuit receiving the first and secondreference currents when the current driving circuit does not receive thefirst and second reference currents responsive to the first and secondselect signals from the first and second phase detector circuits. 16.The phase detector according to claim 5 wherein the charge pumpcomprises an integrator circuit.
 17. A packetized dynamic random accessmemory, comprising: at least one array of memory cells adapted to storedata at a location determined by a row address and a column addressresponsive to a command word; a row address circuit adapted to receiveand decode the row address, and select a row of memory cellscorresponding to the row address responsive to the command word; acolumn address circuit adapted to receive or apply data to one of thememory cells in the selected row corresponding to the column addressresponsive to the command word; a data path circuit adapted to coupledata between an external terminal and the column address circuitresponsive to the command word; and a command data latch circuit forstoring a command data packet at a time determined from a command clocksignal, the command data latch comprising: a latch circuit having a datainput and a clock input, the data input being adapted to receive thecommand data packet and store the command data packet responsive to aclock signal applied to the clock input; and a clock generator circuitfor generating the latch signal from a master clock signal, the clockgenerator circuit comprising: a first delay-lock loop having a firstvoltage controlled delay circuit receiving a reference clock signal andgenerating a sequence of clock signals which are increasingly delayedfrom the reference clock signal to a last clock signal by delaying thereference clock signal by respective delays that are a function of afirst control signal, and a first phase detector comparing the phase ofa first and second clock signal in the sequence and generating the firstcontrol signal as a function of the phase difference therebetween, thefirst phase detector comprising: a first phase detector circuit havingfirst and second input terminals coupled to receive the first and secondclock signals, respectively, and an output terminal, the first phasedetector circuit producing a first select signal having a duty cycleaccording to the phase relationship between a first edge of the firstand second clock signals in the sequence; a second phase detectorcircuit having first and second input terminals coupled to receive thefirst and second clock signals, respectively, and an output terminal,the second phase detector circuit producing a second select signalhaving a duty cycle according to the phase relationship between a secondedge of the first and second clock signals in the sequence; a chargepump having first and second input terminals coupled to the outputterminals of the first and second phase detector circuits and an outputterminal, the charge pump producing a non-varying control signalresponsive to a first combination of logic levels of the first andsecond select signals, an increasing control signal responsive to asecond combination of logic levels of the first and second selectsignals, and a decreasing control signal responsive to a thirdcombination of logic levels of the first and second select signals; anda capacitor coupled to the output terminal of the charge pump; a seconddelay-lock loop having a second voltage controlled delay circuitreceiving the master clock signal and generating a reference clocksignal having delay relative to the master clock signal that is afunction of a second control signal, and a second phase detectorcomparing the phase of the master clock signal to the phase of aselected one of the clock signals in the sequence and generating thesecond control signal as a function of the difference therebetween, thesecond phase detector comprising: a first phase detector circuit havingfirst and second input terminals coupled to receive the master clocksignal and the selected one of the clock signals in the sequence,respectively, and an output terminal, the first phase detector circuitproducing a first select signal having a duty cycle according to thephase relationship between a first edge of the master clock signal andthe selected one of the clock signals in the sequence; a second phasedetector circuit having first and second input terminals coupled toreceived a master clock signal and the selected one of the clock signalsin the sequence, respectively, and an output terminal, the second phasedetector circuit producing a second select signal having a duty cycleaccording to the phase relationship between a second edge of the masterclock signal and the selected one of the clock signals in the sequence;an a charge pump having first and second input terminals coupled to theoutput terminals of the first and second phase detector circuits and anoutput terminal, the charge pump producing a non-varying control signalresponsive to a first combination of logic levels of the first andsecond select signals, an increasing control signal responsive to asecond combination of logic levels of the first and second selectsignals, and a decreasing control signal responsive to a thirdcombination of logic levels of the first and second select signals; anda capacitor coupled to the output terminal of the charge pump; amultiplexer coupled to the first delay-lock loop to receive the clocksignals and couple one of the clock signals to the clock input of thelatch circuit, the clock signal coupled to the latch circuit beingselected by the multiplexer as a function of a select signal applied toa control input of the multiplexer; and a select circuit determiningwhich of the clock signals from the first delay-lock loop should be usedto cause the latch circuit to store the command data packet andgenerating the select signal corresponding thereto.
 18. The packetizeddynamic random access memory of claim 17 wherein the first combinationof logic levels is the first and second select signals at differentlogic levels, the second combination of logic levels is the first andsecond select signals at a high logic level, and the third combinationof logic levels is the first and second select signals at a low logiclevel.
 19. The packetized dynamic random access memory of claim 17wherein the first and second phase detector circuits comprise: a firstsignal transition detector having an input terminal coupled to receive arespective input clock signal and an output terminal, the first signaltransition detector producing a first trigger pulse signal in responseto the respective input clock signal changing logic states; a secondsignal transition detector having an input terminal coupled to receive arespective input clock signal and an output terminal, the second signaltransition detector producing a second trigger pulse signal in responseto the respective input clock signal changing logic states; and aflip-flop having first and second input terminals coupled to the outputterminal of the first transition detector and the output terminal of thesecond transition detector, respectively, and first and second outputterminals coupled to a respective input terminal of the charge pump, theflip-flop set responsive to the first trigger pulse signal and resetresponsive to the second trigger pulse signal.
 20. The packetizeddynamic random access memory of claim 17 wherein the charge pumpcomprises: a first current generator circuit coupled to a firstreference voltage; a second current generator circuit coupled to asecond reference voltage; and a charging circuit coupled between thefirst and second current generator circuits and having first and secondinput terminals coupled to a respective output terminal of the first andsecond phase detector circuits, and an output terminal coupled totransmit the control signal to the output terminal of the charge pump.21. The packetized dynamic random access memory of claim 18 wherein thefirst and second current generator circuits are current mirror circuits.22. The packetized dynamic random access memory of claim 17 wherein thecharge pump is an integrator circuit.
 23. A computer system, comprising:a processor having a processor bus; an input device coupled to theprocessor through the processor bus adapted to allow data to be enteredinto the computer system; an output device coupled to the processorthrough the processor bus adapted to allow data to be output from thecomputer system; and a memory coupled to the processor bus adapted toallow data to be stored, the dynamic random access memory comprising: atleast one array of memory cells adapted to store data at a locationdetermined by a row address and a column address responsive to a commandword; a row address circuit adapted to receive and decode the rowaddress, and select a row of memory cells corresponding to the rowaddress responsive to the command word; a column address circuit adaptedto receive or apply data to one of the memory cells in the selected rowcorresponding to the column address responsive to the command word; adata path circuit adapted to couple data between an external terminaland the column address circuit responsive to the command word; and acommand data latch circuit for storing a command data packet at a timedetermined from a command clock signal, the command data latchcomprising: a latch circuit having a data input and a clock input, thedata input being adapted to receive the command data packet and storethe command data packet responsive to a clock signal applied to theclock input; and a clock generator circuit for generating the latchsignal from a master clock signal, the clock generator circuitcomprising: a first delay-lock loop having a first voltage controlleddelay circuit receiving a reference clock signal and generating asequence of clock signals which are increasingly delayed from thereference clock signal to a last clock signal by delaying the referenceclock signal by respective delays that are a function of a first controlsignal, and a first phase detector comparing the phase of a first andsecond clock signal in the sequence and generating the first controlsignal as a function of the phase difference therebetween, the firstphase detector comprising: a first phase detector circuit having firstand second input terminals coupled to receive the first and second clocksignals, respectively, and an output terminal, the first phase detectorcircuit producing a first select signal having a duty cycle according tothe phase relationship between a first edge of the first and secondclock signals in the sequence; a second phase detector circuit havingfirst and second input terminals coupled to receive the first and secondclock signals, respectively, and an output terminal, the second phasedetector circuit producing a second select signal having a duty cycleaccording to the phase relationship between a second edge of the firstand second clock signals in the sequence; a charge pump having first andsecond input terminals coupled to the output terminals of the first andsecond phase detector circuits and an output terminal, the charge pumpproducing a non-varying control signal responsive to a first combinationof logic levels of the first and second select signals, an increasingcontrol signal responsive to a second combination of logic levels of thefirst and second select signals, and a decreasing control signalresponsive to a third combination of logic levels of the first andsecond select signals; and a capacitor coupled to the output terminal ofthe charge pump; a second delay-lock loop having a second voltagecontrolled delay circuit receiving the master clock signal andgenerating a reference clock signal having delay relative to the masterclock signal that is a function of a second control signal, and a secondphase detector comparing the phase of the master clock signal to thephase of a selected one of the clock signals in the sequence andgenerating the second control signal as a function of the differencetherebetween, the second phase detector comprising: a first phasedetector circuit having first and second input terminals coupled toreceive the master clock signal and the selected one of the clocksignals in the sequence, respectively, and an output terminal, the firstphase detector circuit producing a first select signal having a dutycycle according to the phase relationship between a first edge of themaster clock signal and the selected one of the clock signals in thesequence; a second phase detector circuit having first and second inputterminals coupled to received a master clock signal and the selected oneof the clock signals in the sequence, respectively, and an outputterminal, the second phase detector circuit producing a second selectsignal having a duty cycle according to the phase relationship between asecond edge of the master clock signal and the selected one of the clocksignals in the sequence; an a charge pump having first and second inputterminals coupled to the output terminals of the first and second phasedetector circuits and an output terminal, the charge pump producing anon-varying control signal responsive to a first combination of logiclevels of the first and second select signals, an increasing controlsignal responsive to a second combination of logic levels of the firstand second select signals, and a decreasing control signal responsive toa third combination of logic levels of the first and second selectsignals; and a capacitor coupled to the output terminal of the chargepump; a multiplexer coupled to the first delay-lock loop to receive theclock signals and couple one of the clock signals to the clock input ofthe latch circuit, the clock signal coupled to the latch circuit beingselected by the multiplexer as a function of a select signal applied toa control input of the multiplexer; and a select circuit determiningwhich of the clock signals from the first delay-lock loop should be usedto cause the latch circuit to store the command data packet andgenerating the select signal corresponding thereto.
 24. The computersystem of claim 23 wherein the first combination of logic levels is thefirst and second select signals at different logic levels, the secondcombination of logic levels is the first and second select signals at ahigh logic level, and the third combination of logic levels is the firstand second select signals at a low logic level.
 25. The computer systemof claim 23 wherein the first and second phase detector circuitscomprise: a first signal transition detector having an input terminalcoupled to receive a respective input clock signal and an outputterminal, the first signal transition detector producing a first triggerpulse signal in response to the respective input clock signal changinglogic states; a second signal transition detector having an inputterminal coupled to receive a respective input clock signal and anoutput terminal, the second signal transition detector producing asecond trigger pulse signal in response to the respective input clocksignal changing logic states; and a flip-flop having first and secondinput terminals coupled to the output terminal of the first transitiondetector and the output terminal of the second transition detector,respectively, and first and second output terminals coupled to arespective input terminal of the charge pump, the flip-flop setresponsive to the first trigger pulse signal and reset responsive to thesecond trigger pulse signal.
 26. The computer system of claim 23 whereinthe charge pump comprises: a first current generator circuit coupled toa first reference voltage; a second current generator circuit coupled toa second reference voltage; and a charging circuit coupled between thefirst and second current generator circuits and having first and secondinput terminals coupled to a respective output terminal of the first andsecond phase detector circuits, and an output terminal coupled totransmit the control signal to the output terminal of the charge pump,the charging circuit generating the control signal responsive to thefirst and second select signals from the first and second phase detectorcircuits.
 27. The computer system of claim 26 wherein the first andsecond current generator circuits are current mirror circuits.
 28. Thecomputer system of claim 23 wherein the charge pump comprises anintegrator circuit.
 29. A method of producing a phase adjustment signalto be used by a voltage controlled delay circuit according to the phaserelationship between two input clock signals, the method comprising:determining the phase relationship between the input clock signals;generating a first and second phase information signals according to thephase relationship of the input clock signals; transmitting the firstand second phase information signals to a phase dependent signal source;and generating a non-varying adjustment signal responsive to a firstcombination of logic levels of the first and second select signals, anincreasing control signal responsive to a second combination of logiclevels of the first and second select signals, and a decreasing controlsignal responsive to a third combination of logic levels of the firstand second select signals.
 30. The method of claim 29 wherein the firstcombination of logic levels is when the first and second select signalsare at different logic levels, the second combination of logic levels iswhen the first and second select signals are at a high logic level, andthe third combination of logic levels is when the first and secondselect signals are at a low logic level.
 31. A method of adjusting thephase relationship between two clock signals, comprising: receiving afirst clock signal and a second clock signal having a variable phaserelationship to the first clock signal by the phase detector;determining the phase relationship between the input clock signals;generating first and second phase information signals according to thephase relationship of the first and second clock signals; transmittingthe first and second phase information signals to a phase dependentsignal source; generating a phase adjustment signal having a currentflow of first polarity responsive to the first clock signal leading thesecond clock signal, a current flow of opposite polarity responsive tothe first clock signal lagging the second clock signal, and no currentflow responsive to the first and second clock signals having a knownphase relationship; transmitting the phase adjustment signal from thephase dependent signal source to the voltage controlled delay circuit toadjust the phase relationship between the input clock signals; andvarying the phase delay of the second clock signal with respect to thefirst clock signal to adjust the phase relationship between the inputsignals to the known phase relationship.
 32. The method of adjusting thephase relationship according to claim 31 wherein the known phaserelationship between the input signals is 180 degrees.
 33. The method ofadjusting the phase relationship according to claim 31 wherein the knownphase relationship between the input signals is 360 degrees.